1. Field of the Invention
The present invention relates to an apparatus for forming an image, and more particularly, it relates to an apparatus for forming an image equipped with an intermediate transfer body.
2. Description of the Related Art
In a conventional apparatus for forming an image utilizing an electrophotographic process, image formation is carried out in the following manner. A charged electrophotographic photoreceptor is irradiated with exposure light to form an electrostatic latent image. The electrostatic latent image is developed with a toner, and a toner image thus formed is transferred to a transfer medium, such as paper. The photoreceptor after transferring is subjected to, depending on necessity, removal of the remaining toner with a cleaning device, and then again used in the next image formation process.
The transferring method for the apparatus for forming an image is roughly classified into two methods, i.e., a method where a toner image on the surface of the photoreceptor is directly transferred to a transfer material and a method where a toner image on the surface of the photoreceptor is primarily transferred to an intermediate transfer body, and the primarily transferred image is then secondarily transferred to a transfer material (intermediate transfer method). In the intermediate transfer method among these, it is possible that toner images of prescribed colors (e.g., black, cyan, magenta and yellow) are formed through color separation of a color original image, and the images are transferred as accumulated on the intermediate transfer body to reproduce the color image. Therefore, the method is widely utilized in an apparatus for forming a full color image.
However, it is considerably difficult in the intermediate transfer method that all the toner images of the respective colors are reliably transferred, and there are some cases where the color of the original image is not faithfully reproduced, and nibbled parts are formed on the reproduced image, due to the local transfer failure caused on the primary transferring steps of the respective colors.
In order to avoid such phenomena, accordingly, various measures for improving the transfer property on the primary transferring step have been studied. For example, JP-A-63-194272, JP-A-3-242667, JP-A-4-303869, JP-A-4-303872 and JP-A-5-193020 disclose techniques for improving adhesion between the photoreceptor and the intermediate transfer body by roughening the surface of the intermediate transfer body. JP-A-2-213882 discloses a technique that the linear velocity of the photoreceptor is differentiated from the linear velocity of the intermediate transfer body, and the transfer property on the primary transferring step is improved by both a mechanical force caused by the difference in linear velocity and an electric force by the transferring electric field. Furthermore, such measures are disclosed as a method of controlling the transferring nip pressure (JP-A-1-177063 and JP-A-4-284479, etc.), a method of lowering the wettability of the intermediate transfer body to improve the transfer property (JP-A-2-198476 and JP-A-2-212867), and a method of making the surface tension of the intermediate transfer body larger than the surface tension of the photoreceptor (JP-A-8-211755).
However, it is not always easy that occurrence of nibbled images is sufficiently prevented even by the conventional apparatus for forming an image described in the foregoing. Furthermore, in the method where the surface of the intermediate transfer material is roughened, and difference in linear velocity is provided between the photoreceptor and the intermediate transfer body, wear and damages are liable to occur on the surfaces of the photoreceptor and the intermediate transfer body, whereby the service life of the apparatus is shortened by these methods.
The invention is developed in view of the problems associated with the conventional techniques and is to provide an apparatus for forming an image that can sufficiently prevent occurrence of nibbled images without any damage of the photoreceptor and the intermediate transfer body, whereby good image quality can be stably obtained for a long period of time.
As a result of earnest investigations made by the inventors to attain the invention, it has been found that occurrence of nibbled images in the conventional apparatus for forming an image is ascribed to the combination use of the photoreceptor having a soft surface and the intermediate transfer body having a hard surface, i.e., the difference in surface hardness between the photoreceptor and the intermediate transfer body. Furthermore, as a result of investigations based on the findings, it has also found that the problems are solved by using a combination of a photoreceptor and an intermediate transfer body that satisfy particular conditions of dynamic hardness on the surfaces, and thus the invention has been completed.
Accordingly, the invention provides, as one aspect, an apparatus for forming an image containing an electrophotographic photoreceptor having an electroconductive support having a photosensitive layer formed thereon, a charging unit for charging a surface of the photoreceptor, an exposing unit for exposing the surface of the photoreceptor to form an electrostatic latent image, a developing unit for developing the electrostatic latent image with a toner to form a toner image, and a transferring unit for primarily transferring the toner image to an intermediate transfer body and for secondarily transferring a primary transferred image on the intermediate transfer body to a transfer material, the surface of the photoreceptor having a dynamic hardness of 8xc3x97109 N/m2 or more, a surface of the intermediate transfer body having a dynamic hardness larger than that of the surface of the photoreceptor, and a difference between the dynamic hardness of the surface of the photoreceptor and the dynamic hardness of the surface of the intermediate transfer material being 2xc3x971010 N/m2 or less.
In the apparatus for forming an image according to the invention, an electrostatic latent image is formed by exposure of the charged photoreceptor, and the electrostatic latent image is developed to form a toner image on the surface of the photoreceptor. The toner image is primarily transferred from the photoreceptor to the intermediate transfer body and further secondarily transferred from the intermediate transfer body to the transfer material. When the surfaces of the photoreceptor and the intermediate transfer body satisfy the foregoing conditions in dynamic hardness, at this time, the toner image can be sufficiently and uniformly transferred without any damage on the photoreceptor and the intermediate transfer body. Therefore, according to the apparatus for forming an image of the invention, occurrence of image defects, such as nibbled images, can be sufficiently prevented, and images of good conditions can be stably obtained for a long period of time.
The term dynamic hardness referred to herein means a value measured in the following manner. A microhardness measuring apparatus equipped with a diamond indenter having an edge angle of 115xc2x0 and a tip end curvature radius of 0.1 xcexcm is used, and the diamond indenter is penetrated on the surface of the photoreceptor or the intermediate transfer body at a stress velocity of 0.05 N/sec to measure the penetration depth (m) and the penetration load (N). The dynamic hardness (N/m2) is obtained from the resultant values by using the following equation (a).
DH=3.8584P/D2xe2x80x83xe2x80x83(a)
wherein DH represents the dynamic hardness (N/m2), P represents the penetration load (N), and D represents the penetration depth (m).
It is preferred in the invention that the surface of the intermediate transfer body has a dynamic hardness of 1.8xc3x971010 N/m2 or more. When the dynamic hardness of the surface of the intermediate transfer body satisfies the conditions, occurrence of image defects, such as nibbled images, can be further sufficiently prevented.
It is also preferred in the invention that the photosensitive layer of the photoreceptor has a protective layer on a farther side from the support, and when the protective layer and the intermediate transfer body are formed on glass supports, respectively, the surface of the intermediate transfer body has a dynamic hardness larger than a dynamic hardness of a surface of the protective layer, and a difference between the dynamic hardness of the surface of the protective layer and the dynamic hardness of the surface of the intermediate transfer body is 1.5xc3x971010 N/m2 or less. According to the configuration, occurrence of nibbled images and damages on the photoreceptor and the intermediate transfer body can be further sufficiently prevented. The term dynamic hardness referred to in this embodiment means the same value in terms of N/m2 referred in the foregoing except that the stress velocity is 0.09 N/sec.
It is also preferred in the invention that the protective layer contains a polysiloxane resin having charge transporting property and a crosslinked structure. When a polysiloxane resin is contained in the protective layer, the mechanical and electrical strength of the surface of the photoreceptor is improved, whereby the service life of the apparatus for forming an image of the invention can be prolonged.
It is also preferred in the invention that the intermediate transfer body contains a polyimide resin. When a polyimide resin is contained in the intermediate transfer body, the mechanical and electrical strength of the intermediate transfer body is improved, whereby the service life of the apparatus for forming an image of the invention can be prolonged. Furthermore, in the case where plural color toners having different colors are used in the apparatus for forming an image of the invention, color drift of the toner images of the respective colors can be certainly prevented, whereby reproducibility in color can be improved.
The invention also provides, as another aspect, a process for forming an image containing steps of: charging an electrophotographic photoreceptor having a surface having a dynamic hardness of 8xc3x97109 N/m2 or more; exposing the charged surface of the electrophotographic photoreceptor to form an electrostatic latent image; developing the electrophotographic latent image with a toner to form a toner image; primarily transferring the toner image to an intermediate transfer body having a surface having a dynamic hardness larger than that of the surface of the photoreceptor, with a difference between the dynamic hardness of the surface of the photoreceptor and the dynamic hardness of the surface of the intermediate transfer material being 2xc3x971010 N/m2 or less; and secondarily transferring a primary transferred image on the intermediate transfer body to a transfer material.